Add extra path validation tests for EKU processing

net/data/verify_certificate_chain_unittest/intermediate-eku-clientauth/generate-chains.py

 #!/usr/bin/python
 # Copyright (c) 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Certificate chain with 1 intermediate and a trusted root. The intermediate
 restricts the EKU to clientAuth, and the target has serverAuth +
 clientAuth. Verification is expected to fail when requesting serverAuth."""
 
 import sys
 sys.path += ['..']
 
 import common
 
 # Self-signed root certificate (used as trust anchor).
 root = common.create_self_signed_root_certificate('Root')
 
 # Intermediate certificate.
 intermediate = common.create_intermediate_certificate('Intermediate', root)
 intermediate.get_extensions().set_property('extendedKeyUsage',
                                            'clientAuth')
 
 # Target certificate.
 target = common.create_end_entity_certificate('Target', intermediate)
 target.get_extensions().set_property('extendedKeyUsage',
                                      'serverAuth,clientAuth')
 
 chain = [target, intermediate, root]
 common.write_chain(__doc__, chain, 'chain.pem')